One type of cellular communication system employs a technology known as CDMA (code division multiple access). In this system, users are distinguished from one another by unique codes assigned to them while sharing the same frequency band at the same time. Since all the users are using the same frequency band, users interfere with each other. The level of interference must be controlled to provide the intended quality of service, that is, to reduce the rate of errors below an acceptable level. For further description of CDMA and the differences between CDMA, TDMA (time division multiple access), and FDMA (frequency division multiple access), please see "Code Division Multiple Access," Communications, February 1990, by Fred Baumgartner, hereby incorporated by reference.
FIG. 1 graphically illustrates a CDMA-based communication system comprising a mobile station 10, a base station 20, reverse link 30 which represents the electromagnetic wave communication link transmitted from mobile station 10 to base station 20 and forward link 40 which represents the electromagnetic wave communication link transmitted from base station 20 to mobile station 10.
One method to control the level of interference in a CDMA-based communication system is by way of power control, that is, the power transmitted from the mobile station 10 to the base station 20 (reverse link) or from the base station 20 to the mobile station 10 (forward link) is controlled or varied. Power control on the reverse link is required because of the so-called "near/far" problem. The problem occurs when a mobile station close to a base station undergoes or incurs relatively low power propagation loss, and thus the base station sees a very high level of power from the mobile station, whereas a mobile station located farther out undergoes relatively high propagation loss, and thus the power received by the base station from the mobile station is very low. Power control for the reverse link is thus used to command the nearby and far-out mobile stations to reduce or increase the transmit power to the base station, as is well known in the art.
Due to the variability in the propagation environment as the mobile stations move about, the power control command update rate must be relatively fast. The speed at which the power command update rate occurs in present IS-95 CDMA-based systems is 800 hertz. In past CDMA-based communication systems, it was deemed that the power control on the forward link was not as critical as the power control on the reverse link, and a relatively slow power control update scheme was employed (typically less than 1 hertz). In a recent PCS (personal communication system)standard J-STD-008, a power control update scheme on the forward link was adapted which allows an update rate of 50 hertz. The electromagnetic wave phenomena known as fast fading cannot be adequately addressed by a forward link power control update scheme operating at relatively slow rates, such as less then 50 hertz. This has resulted in requiring relatively high power for forward traffic channels to reduce errors below an acceptable rate. However, raising power for the forward traffic channels reduces the forward channel capacity significantly.
To improve the above problems, a CDMA standard, W-CDMA, has recently been proposed to increase the speed of the forward power control up to 800 hertz. A mobile station estimates the quality of the forward link in a well known closed loop power control scheme and sends forward power control command bits to one or more base stations depending on whether the mobile station is in a handoff state or not. These power control command bits are sent at a rate of 800 hertz and are sent via the control channel of the reverse link.
In the proposed W-CDMA standard, the reverse channel has two major subchannels: the access channel and the traffic channel. The traffic channel, in turn, has four subchannels: the pilot channel, the fundamental channel, the supplemental channel, and the control channel. In the proposed W-CDMA standard, the same power control bits are sent to all base stations when the mobile station is in the handoff state. Thus, the W-CDMA standard power control scheme does not allow independent base station power control when a mobile station is in a handoff state. This results in some base stations transmitting more power than needed. As a consequence, the interference level to other mobile stations caused by forward link increases and thus results in a lower channel capacity.
The current proposed control channel structure implemented in the mobile station for the W-CDMA standard is illustrated in FIG. 2. Ten bits of control information relating to the fundamental and supplemental channels are sent to a cyclic redundancy checker (CRC) 50 to detect error states. The output of the CRC 50 is fed into an encoder tail 60. The output of the encoder tail 60 is sent into a convolutional encoder 70. The output of the convolutional encoder 70 is sent into a mux 80. Sixteen power control bits (PCBs) per 20 milliseconds or, 1 power control bit (PCB) per 1.25 milliseconds is fed into repetition block 90. The output of the repetition block is sent into mux 80 so that there are three power control bits for every information bit, and the output the mux 80 is 384 bits. For further description of the control channel structure and the components comprising the control channel structure, please see TIA/EIA/IS-95A Standard (herein incorporated by reference).
What results is a control channel structure which does not allow independent base station power control, because the power control command bits sent to the base stations are generated based on the aggregated quality of two or more signals received from multiple base stations.
What is needed is a new fast forward power control scheme for CDMA-based communication systems which allows optimized, individual base station power control. This provides a reduced interference level on the forward link which, in turn, increases channel capacity for the forward link.